Graphite-based structures, e.g. graphene quantum dots, graphene nanoribbons (GNRs), graphene nanonetworks, graphene plasmonics, and graphene super-lattices, exhibit many exceptional chemical, mechanical, electronic and optical properties, and are desirable for use in electronic devices, composite materials, and energy generation and storage. Such graphite-based structures in general comprise one or more graphene layers that are in electrical communication with one or other, or with an external circuit. Each graphene layer is typically nanometers thick and has a characteristic dimension (e.g., length or width) that also is in the nanometer range. Connecting such graphene layers, in particular graphene layers with horizontally oriented graphene sheets, imposes great challenges in the development of fabrication methods that can produce graphite-based structures with consistent and reliable lead connections to achieve desired functionalities.
Current methods deposit leads after graphene growth or exfoliation. To form a lead connection with a graphene layer, current methods typically rely on lead diffusion or migration into the graphene layer. In some cases, current methods also require additional processes to facilitate the formation of a lead connection. Graphite-based structures and devices formed by such methods experience many problems including lead connection integrity, additional complex processes, loss of functional graphene surface, as well as structural or functional failure of the graphite-based devices.
One drawback with the current methods is related to the integrity of the lead connections. For example, a lead deposited on a graphene layer having horizontally oriented graphene sheets forms a lead that is usually not in full contact with all of the graphene sheets in the graphene layer. Instead, the lead is generally in contact only with the top graphene sheet. To form lead connection with the underlying graphene sheets, the lead is usually heated to assist lead diffusion into the graphene layer. One problem with this process exists in the difficulty of controlling the size, shape or depth of the lead because diffusion occurs in all directions, e.g., diffusing down through the graphene sheets as well as along the graphene sheets. Another problem with this process is the potential changes in structural, physical or chemical properties of the graphene layer as the lead migrates into the graphene layer. For example, the graphene layer may be oxidized under heat during the lead diffusion. In addition, a lead contact may be converted from an ohmic contact to a non-ohmic contact during this process.
Another drawback with current methods is related to the complex processes that are required for the formation of lead connection. Because the current methods deposit leads after graphene growth or exfoliation, the graphene layer has to be isolated or etched into specific geometries before the deposition of a lead in order to achieve desired functionality. However, graphene can be difficult to process because maintaining selectivity when etching carbon-based materials is difficult in relation to other materials. In addition to etching, several other processes are required using the current methods for the formation of the lead connection. These processes include the deposition of the lead after etching and processes that facilitates adhesion and migration of lead into the graphene layer.
The required etching process before lead deposition imposes further concerns on the current methods: the loss of the functional graphene surface and unsatisfactory packing density. The ability to pack structures and devices onto a surface with high packing density and larger functional graphene surfaces is an important issue, because packing density and functional graphene surfaces determine functionalities of devices such as efficiency of solar cells or detectors. However, current techniques use horizontal isolation, resulting in the loss of the functional graphene surfaces.
There are other drawbacks with current manufacturing methods. In the first instance, horizontal leads typically have a width that is 500 nm or larger. What is desired, however, are leads that are 3 nm to 10 nm wide. However, connecting leads that are only 3 nm to 10 nm wide to an external circuit generally requires a large bonding pad area. Because of this and other problems, such as those described above, the graphite-based devices fabricated by the current methods are prone to structural or functional failures were they to be used to incorporate leads that are only 3 nm to 10 nm wide.
Given the above background, there is a need in the art for improved fabrication methods that can produce consistent and reliable graphite-based structures with interconnected leads and graphene layers. There is further need in the art for improved fabrication methods that time can produce interconnected leads that have widths that are in the nanometer range (e.g., having widths of less than 100 nm), rather than in the micrometer range.